There's lots of these errors of understanding throughout the talk, but the biggest one is the rolodex card icon. It looks like an ID card more than a rolodex card, since the latter do not have portraits on them. Check it out on a google image search; some companies make cards for you with their logo on it, but most cards are blank for your writing, and there is no portraiture. Presented with that icon, photo-on-the-left, text-on-the-right looks like a pretty default ID card. The underlying point is right - that the teenagers recognise the meaning of the icon - the but rationalisation for why is incorrect.
So why do teenagers understand that floppy means save? Because 'convention'. Not because "they're smart, and backward-map old technology". If we had a different icon for 'save', and one company just today started using the floppy, then no-one would understand it.
Using the word "meaning" here is problematic because it comes with some intellectual baggage (e.g, connotation). What does anything "mean" really? The most straight forward answer to me is it "means" what it does. That is, function is meaning, especially in UX.
The unnecessarily complexity is what's in the video: the user sees the icon, recognises it as a floppy disk, understands that the floppy disk is storage from some prior experience (ha![1]), and then reverse-engineers the question "why does this icon look like a floppy disk" into "something to do with storage - probably saving the document to it". It's a very convoluted and unnecessary cognitive path.
[1] Does anyone really believe that the youth of today learned what floppy disks were before they learned about saving documents?
It's about training. Look at this picture[1]. If you made an icon from this simple structure, do you think people would understand what it meant? It's a pretty unknown bit of technology, but what it does has a clear analogue to a common function on websites. If only one site used it, it would be confusing. But if it were used by almost everyone, all through your growing up, you'd both understand the icon, and that it was shaped that way because it's meant to look like this object.
[1] http://www.reon-tuellensiebe.de/_EN/kategorien/Home/dateien/...
Iconifying things is hard.
Although being honest, I got impressed by the "tape" icon used for voicemail.
also I do think a nicely simplified telegraph key could be an excellent 'send' or 'communication' icon
